1. Field of the Invention
The invention pertains to apparatus and methods to treat carbon and graphite materials, and more particularly to systems and methods for plasma assisted treatment of carbon fibers to improve their surface characteristics for use in polymer matrix composites.
2. Description of Related Art
Carbon fiber composites require that the carbon fibers be bonded to a resin to form a structurally sound composite. Virgin (untreated) carbon fibers are well known to have a low chemical affinity to resins and other polymeric based materials. Usually the carbon fiber must be treated to increase the amount of chemical affinity to the matrix resin. The adhesive characteristics of the resin to the carbon fiber are dominated by the surface energy of the carbon fiber. Through surface treatment, the addition of oxygen atoms to the surface of the carbon fiber has been demonstrated to increase the surface energy of the carbon fiber. Standard industrial processes for increasing the surface energy of carbon fiber are the use of gaseous oxidative species (ozone) or a liquid electrostatic technique. These common techniques result in bound oxygen concentrations of ˜6%. There are other techniques, i.e. chemical coupling agents, liquid oxidizing agents, low pressure gas plasma, chemical solutions, vapor phase deposition, chemical etching, and electropolymerization; however these processes are not commonly used in industry.
The use of oxygen-containing plasmas to modify the surface of carbon fibers has been previously examined. The advantages are: increasing the level of oxygen onto the surface; increasing the surface energy; providing an oxidative treatment of the top surface creating a different type of oxidative chemical groups; and improving adhesion with resin by improving the chemical affinity of the resin and fiber surface. The disadvantages are: undesirable mechanical surface effects; surface ablation, etching, erosion, or pitting leading to the creation of localized surface irregularities on the carbon fiber; removal of the outmost structures and morphology; general structural damage to the fiber; and reduced mechanical strength.
U.S. Pat. No. 6,514,449 teaches the use of microwave energy and plasma to modify the surface topography of carbon fiber. General discussions of fiber surface modification by plasma processing may be found in several references [Mittal, K. L. and Pizzi, A., “Adhesion Promotion Techniques. Technological Applications,” Marcel Dekker, pp. 67-76 and pp. 139-73. (1999); J. B. Donnet, T. K. Wang, S. Rebouillat and J. C. M. Peng, “Carbon Fibers,” Third Edition, Marcel Dekker, Inc., pp. 180-9 (1998)]. These teachings address the modification of surface morphology or surface chemistry as a means of modifying interactions between fiber and matrix in a composite.
Low-pressure (3-5 Torr) plasma processing has been described for several different operations relating to carbon fiber production (see Paulauskas et al., U.S. Pat. No. 6,372,192), In this patent it was suggested that small amounts of oxygen could be admitted into a low-pressure microwave plasma to achieve some surface treatment of the fibers.
It is widely recognized that low-pressure plasma processes are less desirable to industrial operations because of the high cost, space, and maintenance requirements associated with vacuum systems [see C.-M. Chan, Polymer Surface Modification and Characterization, Hanser Pub., 1994, pp. 225-63]. A viable plasma-based process that could operate at near ambient pressures while achieving adequate modification of the fibers was therefore needed and was not available based on prior teachings in the literature.